Can More Frequent Testing Improve I/M Program Effectiveness? Analysis ofthe Effect of Annual Testing in the Phoenix IM240 Program

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In this report we examine the potential for additional emission reductions from adopting annualrather than biennial testing in vehicle emission inspection and maintenance (I/M) programs. Wetracked a fleet of cars reporting for testing in two biennial cycles in the Phoenix IM240 program.Using an assumption of linear deterioration following a passing I/M test, we found that annualtesting in the Phoenix program in 1996 would have resulted in an additional 45%, 48%, and 27%reduction in HC, CO, and NOx emissions, respectively, over what occurred in the biennialprogram. More stringent cut points under a biennial program would have resulted in greateremission reductions, but with higher fail rates. Tighter cut points can also be applied to theannual program. In fact, an annual program might ease the progression to tighter cut points byresulting in relatively lower failure rates per test than biennial programs.

A small number of cars were identified that were given three I/M cycles, each roughly one yearapart, in Phoenix. This group of vehicles had higher overall emissions and may not berepresentative of the whole fleet. However, these cars appear to have the same HC and COemissions, but slightly lower NOx emissions, after two years as cars tested biennially. Thisresult suggests that our assumption that emissions after annual testing would deteriorate at thesame rate as observed in biennial testing may be optimistic. The findings from our analysessuggest that test-to-test emissions variability is a limitation of existing I/M programs, and ispreventing them from properly identifying some vehicles with broken or malfunctioningemissions controls and ensuring that they are repaired. In theory, more frequent testing ofsuspected high emitters could help address this problem. One strategy that may work is to useremote sensing to identify suspected high emitters, and require that they come in for off-cycletesting, as frequently as necessary.

Arizona remote sensing data provide additional evidence that a significant benefit results fromannual testing of older cars. Remote sensing measurements made in Phoenix of vehicles testedbiennially under the Phoenix enhanced IM240 program and of vehicles tested annually under theTucson two-speed idle program show interesting results. While newer vehicles from Phoenixhave lower emissions than those from Tucson, as one might expect from the enhanced program,the older vehicles from Phoenix have higher emissions than those from Tucson. Phoenixvehicles over seven years old had emissions 37% to 47% higher than Tucson vehicles of thesame age.

We conclude that more attention should be paid to promptly identifying and properly repairingthe high emitters found in the older portion of the vehicle fleet.

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